Along with gem stones (e.g., diamond, ruby, emerald, topaz, opal, jade), and precious metals (e.g., gold, silver, platinum), pearls are among the most prized possessions (or luxury items) for human beings for millenniums. Beside their natural beauty, the brilliant color and luster, they are often associated with social status and level of well-being. As a result, and not surprisingly, the trend of cosmetics makeup is to emulate or recreate these “natural” and “aesthetic” appearances of pearl, gem and precious metals with less expensive materials such as interference pigments (e.g., metal oxide coated mica). The most common types of effect pigments are micronized titanium dioxide, metal oxide coated mica, metal oxide coated alumina, metal oxide coated silica, basic lead carbonate, bismuth oxychloride, and natural fish silver.
Metal oxide coated mica pigments are characterized by excellent optical, chemical, mechanical, toxicological, and environmental properties. Natural or synthetic mica, and alternative supports, such as aluminum flakes, or SiO2 platelets, can be used alone, or as a support for titanium dioxide, iron oxide (Fe2O3 or Fe3O4), iron ferrocyanide (Iron Blue or Prussian Blue), tin oxide, and chromium oxide. The color space defined by these coated mica-based pigments is based on the type of coating (e.g. metal oxide, colorant, etc.) used, the layer thickness, and the number of coated layers.
Among the natural pearls, the most expensive are black pearls, which come with various undertone and color flops. To faithfully emulate this aesthetic optical effect in cosmetic makeup is one of the top challenges facing a cosmetic pigment maker and formulator. The traditional approach to these pigments is to blend dark solid-color inorganic pigment (e.g., black iron oxide or carbon black) with white platy pearlescent pigments (e.g., TiO2 coated mica, TiO2 coated borosilicate, TiO2 coated alumina). The platy interference pigment provides the luster, brilliance (reflection), transparency and depth of field. The solid-color pigment(s) provide(s) the dark undertone and surface coverage. However, this type of blend usually appears to be much “dirtier”, “lack luster”, and “lack transparency” compared to the natural pearl. The primary reason for that is fouling of the smooth surface of white pearlescent pigment by the solid-color pigment granules, which leads to light scattering and disruption of light interference.
Metal oxide coated platelet pigments may be magnetic or exhibit magnetic susceptibility. When placed into a liquid coating, regions of the coated pigment may be aligned by an externally applied magnetic field and produce a goniochromatic, or angle dependent optical effect. This effect may be used to create an impression of a two- or three-dimensional image. After the pigments have been aligned, the coating may be cured to solidify the optical effect. Examples of pigments and methods of aligning them are discussed in U.S. Pat. Nos. 6,589,331; 6,902,807; 5,223,360; 6,759,097; and 7,258,900.
The use of metallic colored (copper, bronze, maroon/russet shades, gold, etc.) pearlescent pigments is widespread and can be applied in fields such as decorative cosmetics, plastics, advanced security printing and automotive and industrial coatings. Commercial products currently available include: Iriodin®, Xiralic®, Timiron®, Xirona® and Colorona® lines by Merck, Cloisonné® and Timica® lines by BASF, SunPearl® and Sunshine® lines by SunChemical. Metallic pearlescent shades (bronze, copper, russet, etc.) contained within these lines of pigments are generally developed by deposition of α-Fe2O3 (hematite) on the surface of a platelet-like substrate, such as mica, Al2O3 platelets, calcium borosilicate, or other laminar substrates. Single-layer pigments of this type have a yellow-red absorbance color combined with an interference color directly related to the thickness of the iron oxide layer. This combination of light reflection, absorption and interference can be utilized to produce lustrous, pearlescent effects ranging from gold to deep maroon shades. In addition, given that iron oxides absorb a portion of the incident light, these pigments are defined by intermediate hiding power relative to transparent TiO2-coated pearlescent pigments and opaque metallic effect pigments (such as aluminum flake).
Pearlescent pigments comprised of a platelet-like substrate coated with α-Fe2O3 generally have magnetic mass susceptibility values in the vicinity of 0.01 to 0.02×10−5 m3/kg. Thus, these pigments are not easily applicable to printing methodologies that utilize an external magnetic field to manipulate pigment orientation such as those described in U.S. Pat. Nos. 5,223,360; 6,645,286; and 6,759,097. Platelet-like substrates coated with Fe3O4 (magnetite) are defined by a much higher magnetic susceptibility. For instance, Colorona® Blackstar Red, and Blackstar Gold have magnetic mass susceptibility values of 11.56 and 11.08×10−5 m3/kg, respectively. These types of pigments can be used in magnetically-aligned coating applications; however, they are generally confined within a very narrow color space (dark shades or dark shades with muted inference colors).
Consequently, a significant need exists for pigments with higher magnetic susceptibility, more colors, and are easier to manufacture.